Halogen-Dependent Deep-Blue Circularly Polarized Emitters with Ultrahigh-Color-Purity Under High Pressure

Abstract Achieving free exciton (FE) emission in low-dimensional (2D, 1D, and 0D) metal halide perovskites is challenging due to the strong electron–phonon coupling effect induced by lead halide octahedral distortion. Herein, the FE emission behaviors of three new 2D chiral perovskites, (R-3-XPEA)₂PbBr₄ (PEA = phenethylamine, X = F, Cl, Br), were investigated under hydrostatic pressure. (R-3-BrPEA)₂PbBr₄ and (R-3-ClPEA)₂PbBr₄ exhibited high color–purity deep-blue circularly polarized luminescence (CPL) dominated by FE at pressures of 1.7 and 2.5 GPa, respectively, whereas (R-3-FPEA)₂PbBr₄ presented broadband warm-white CPL under high pressure. The structural analysis and theoretical calculation results demonstrated that pressure reduced the penetration depths of R-3-BrPEA⁺ and R-3-ClPEA⁺ into [PbBr₆]⁴⁻ inorganic frameworks by strengthening halogen···halogen (Br···Br and Cl···Cl) interactions between organic amines, resulting in smaller [PbBr₆]⁴⁻ octahedral distortion and weaker electron–phonon coupling in (R-3-BrPEA)₂PbBr₄ and (R-3-ClPEA)₂PbBr₄. Thus, pressure-driven enhancement of halogen···halogen interactions was responsible for remarkable deep-blue CPL in (R-3-BrPEA)₂PbBr₄ and (R-3-ClPEA)₂PbBr₄. Conversely, [PbBr₆]⁴⁻ octahedral distortion and strong electron–phonon coupling could not be effectively suppressed in (R-3-FPEA)₂PbBr₄ owing to the lack of halogen···halogen interactions, leading to the absence of deep-blue CPL. Our work gives a new insight into the intrinsic structure-property relationship between noncovalent interactions and the ultrahigh-color-purity emission behavior in chiral perovskites.